1. Field of the Invention
This invention relates to flow cytometers and hematology analyzers, and, more particularly, to hematology analyzers that count and identify biological cells using light scattering and fluorescence techniques in an optical flowcell.
2. Discussion of the Art
Flow cytometry is a technique for counting, examining, and sorting microscopic particles suspended in a stream of fluid. Flow cytometry allows simultaneous, multiparametric analysis of the physical and/or biochemical characteristics of single cells flowing through an optical/electronic detection apparatus. When used in hematology analyzers, flow cytometry enables the precise counting of cells in a measured volume of blood or other biological fluid sample and the identification of those cells based on the use of light scattering and/or fluorescence detection. As used herein, the phrase “flow cytometry” refers to the techniques and apparatus used in flow cytometers as well as in flow-cytometry-based hematology analyzers and other diagnostic instruments.
In flow cytometry, a beam of light, such as, for example, laser light of a single wavelength, light of a broader spectral nature from a light-emitting diode (LED), or some other source of light, is directed onto a hydrodynamically focused stream of a fluid carrying particles, or onto such a stream otherwise confined. A number of detectors are aimed at the region where the stream passes through the light beam, one or more detectors being in line with the light beam and typically several detectors positioned perpendicular to the light beam. The detector(s) in line with the light beam detect forward scatter, in one or more angular annuli or regions, or optical extinction, or both forward scatter and optical extinction. The detectors positioned perpendicular to the light beam detect side scatter, fluorescence, or both side scatter and fluorescence. Each suspended particle passing through the beam scatters the light in some way, and fluorescent chemicals in, or on, the particle, and either natively present in the particle or added to it during an incubation step, may be sufficiently excited to emit light at a longer wavelength than that of the light source. The combination of absorption, scattered light, and fluorescent light is detected by the detectors, and by analyzing fluctuations in intensity at each detector (typically one detector for each desired fluorescent emission band and one detector for each annulus or region of scattering angles), it is possible to determine various facts about the physical and biochemical structure of each individual particle. Forward scatter correlates with the volume of the cell and side scatter depends on the complexity of the particle, such as, for example, the shape of the nucleus, the amount and type of cytoplasmic granules or the roughness of the cellular membrane. Fluorescent markers can be conjugated with monoclonal antibodies that selectively bind to antigens present on certain types of cells or to cells in a particular pathological state; fluorescent dyes that bind selectively to nucleic acids in either the cytoplasm, cellular nucleus, or both, may also be employed. Representative examples of instruments employing flow cytometers are described in U.S. Pat. Nos. 5,017,497; 5,138,181; 5,350,695; 5,812,419; 5,939,326; 6,579,685; 6,618,143; and United States Patent Publication No. 2003/0143117 A1. These documents describe a flowing stream of cells and a stationary beam.
A subfield of cytometry, laser scanning cytometry (LSC), involves scanning a laser beam across a field of interrogation. However, the field of interrogation is stationary, typically a section of a microscope slide to which cells have been adhered, and the measurement rate (i.e., the number of cells analyzed in a given unit of time) obtainable through such a scheme is far below what can be obtained by conventional flow cytometry. Furthermore, LSC is an imaging method suitable for detailed analysis of a relatively limited number of cells, whereas flow cytometry is a light-scattering and fluorescence-tagging method of analyzing large quantities of cells. See, for example, U.S. Pat. Nos. 5,072,382, 5,523,207, and 6,002,788. Two other techniques closely related to LSC are volumetric capillary cytometry (see, for example, U.S. Pat. No. 5,962,238 and European Patent No. 0681/78) and microvolume LSC (see, for example, U.S. Pat. Nos. 6,603,537 and 6,687,395, and United States Patent Publication No. 2005/0280817). All of these techniques rely on a scanning laser beam impinging upon a specimen fixed to a controllable stage and on methods based on highly resolved imaging, confocal scanning, or spectroscopy techniques.
Several teachings in the prior art describe an imaging flow cytometer that combines the flow characteristics of a conventional analyzer with imaging capabilities. See, for example, U.S. Pat. Nos. 5,083,014, 5,444,527, 5,521,699, 5,644,388, 5,824,269, 6,671,044, and 6,975,400, and United States Patent Publication Nos. 2002/0146734 and 2002/0057432. In the prior art, (a) the laser or other light source is stationary, necessitating the use of a charge-coupled detector (CCD) array in order to capture information from across the field of interrogation; and (b) the information obtained is of an imaging nature rather than of a scattering nature. This approach causes the process to run significantly more slowly than in flow cytometry; in other words, in order to obtain more detailed information for each cell by the use of the disclosed imaging strategy, the measurement rate is reduced, i.e., the overall number of cells actually analyzed in a given unit of time is reduced.
One of the key advantages of imaging methods is that such methods are capable of capturing fine details of individual cells, which enable a trained professional to make positive identifications in borderline cases. However, the greater detail obtainable by imaging methods are balanced by the reduction in the total number of cells that can be analyzed in this way in a given period of time. In methods based on scattering, identification is based on characteristics that are averaged over the cell (such as cell size, hemoglobin content, lobularity of the nucleus, etc.); however, the loss of fine detail in individual cells is compensated for by the ability to collect desired information for tens of thousands of cells in a matter of seconds. Such information can be used to plot the results in aggregate according to a few characteristics (such as, for example, size, lobularity, etc.).
The CELL-DYN® Sapphire® hematology analyzer (commercially available from Abbott Laboratories), an instrument based in part on flow cytometry, processes a minimum of 105 complete blood count (CBC) samples per hour under standard conditions. This aspect of performance is referred to as the throughput of the instrument. Other commercially available hematology analyzers are capable of processing up to 150 standard CBC samples per hour, although the performance tradeoffs adopted in their designs usually result in higher rates of reflex testing, slide review, or both reflex testing and slide review. It would be desirable to increase the effective throughput of hematology analyzers (i.e., accounting for both the mechanical throughput and the rate of first-pass reportability) so as to be able to process a higher volume of standard CBC samples per hour than currently possible, while at the same time maintaining a low rate of reflex testing and slide review. This improvement would enable use of such an analyzer in a high-volume laboratory (reference laboratory or hospital core laboratory), which requires the processing of large numbers of standard, mainly normal, CBC samples per day with as few slide reviews as possible. It would also enable higher throughput of samples in any of the other laboratory environments where an analyzer is used.
There are several obstacles to higher throughput, such as, for example, loading samples, aspirating samples, dispensing samples, diluting samples, mixing samples, incubating samples, staging samples, delivering samples to the flowcell, and the time required for a sequential measurement of a series of samples. These obstacles can be thought of as bottlenecks, where the narrowest bottleneck determines the overall throughput of the instrument. The current narrowest bottleneck in the CELL-DYN®Sapphire® instrument is the time involved in the sequential measurements through the optical flowcell. The performance currently achieved involves a compromise between acceptable levels of coincidences, acceptable precision of results (total number of cells counted), constraints from the present hardware/electronics architecture, i.e., arrangement of hardware and electronic components, and constraints from the assay strategy involving reagents and dilution. As used herein, a “coincidence” is interpreted to mean an event where two or more cells, either of a similar type or a dissimilar type, are sufficiently close that they cannot be resolved by the instrument, are counted as one, and are misidentified in one or more detection parameters.
Increasing the flow rate through the flowcell by widening the sample stream, by increasing the velocity of the sample stream, or both of the foregoing, have all been attempted. In a conventional flow cytometer, where the sample stream is intersected by a stationary beam, the measurement rate in the linear regime (defined as the number of cells being analyzed per second, n) is given byn=ρxstreamzstreamvsteam,  (Eq. 1)where ρ represents the concentration of cells in the sample stream, xstream represents the transverse dimension of the illuminated portion of the sample stream, zstream represents the longitudinal dimension of the illuminated portion of the sample stream, and vstream represents the flow velocity. In order to increase the measurement rate, one can attempt to increase any one of those four quantities. However, under the circumstances encountered in the state of the art, increasing ρ leads to greater coincidence events, as does increasing xstream and zstream. Increasing vstream can lead to risks related to the onset of turbulence or other kind of hydrodynamic instability, which can severely reduce the precision of the measurements, because the resulting sample stream oscillates or fluctuates unpredictably across a stationary light beam.
Other options include simply doubling the entire measurement hardware, with two sets of measurements occurring in parallel on separate flowcells interrogated by separate sources of light. Two sources of light can be employed or a single source of light can be split into two. The shortcomings of this approach are increased complexity, a greatly increased cost, a greatly increased risk to reliability because of the large number of additional components, and increased service costs.
U.S. patent application Ser. No. 11/934,277, incorporated in full herein by reference, addresses satisfactorily the issues described above, namely improving the throughput of a flow cytometer without incurring higher coincidences, without degrading precision of results, without greatly changing the hardware and/or electronics (and consequently having to meet most of the same constraints), without necessarily changing the chemistries and dilutions currently in use, and while maintaining the currently available desirable attributes associated with a high rate of first-pass reportability of results. That disclosure describes a method and apparatus capable of achieving a significant improvement in performance with relatively limited changes in the architecture and operation of a current analyzer. While such limited scope of design changes is attractive and beneficial from a commercial viewpoint, it also constrains the degree to which the innovations described in the concurrent disclosure can be exploited.
In hematological assays aimed at determining parameters from human whole blood, there are two physiological factors that present obstacles to simple, rapid, and accurate determination of cell counts. One factor is that, in typical fresh peripheral human whole blood, there are about 1,000 red blood cells (RBCs) and about 50 platelets for each white blood cell (WBC). The other factor is that, while platelets are typically sufficiently smaller than any other cell type to allow discrimination based on size, and most white blood cells (WBCs) are sufficiently larger than either RBCs or platelets to again allow discrimination based on size, two cell species in particular—RBCs and lymphocytes, a subtype of WBCs—typically overlap in size distribution (as well as in their scattering signatures) to a sufficient degree to make discrimination based on size prone to gross error. Therefore, when determining RBCs mainly by size discrimination, the asymmetry in concentration works in one's favor, since the occasional WBC misclassified as a RBC will not, generally, affect the overall accuracy of the measured concentration of RBCs to any appreciable degree; however, the converse is not true, and any unaccounted for interference from RBCs in determining the concentration of lymphocytes (and, by extension, the overall concentration of WBCs) would yield very inaccurate results.
Consequently, methods have been developed in the prior art to handle this large asymmetry and size overlap and still provide useful results in an acceptable time frame. One standard method employed in the prior art has been to separate the blood sample to be analyzed into at least two aliquots, one destined for RBC and platetet analysis, and one for WBC analysis. The aliquot destined for WBC analysis is mixed with a reagent solution containing a lysing reagent that preferentially attacks the membranes of the RBCs. Partially on account of their loss of hemoglobin through the compromised membrane, and partially on account of their attendant reduction in size, the resulting lysed RBCs become distinguishable from lymphocytes based on their respective scattering signatures. Another method employed in the prior art involves using nucleic acid dyes to provide a fluorescent distinction between the RBCs and the WBCs. WBCs contain a nucleus containing DNA. When these WBCs are labeled via a fluorescent label, they can be distinguished from mature RBCs, whose nuclei have been expelled in the maturation process.
Both of these methods have drawbacks. First of all, the lysing reagent used to dissolve the RBCs can attack the WBCs as well, reducing their integrity and eventually dissolving them, too. This is particularly a problem with WBCs that are already fragile in the first place, due to some pathological condition (such, as, for example, chronic lymphocytic leukemia). At the other end are types of RBCs (such as, for example, those found in neonates, and in patients with thalassemia, sickle-cell anemia, and liver disease) which are naturally resistant to lysis, and which therefore tend to persist as interferents in WBC assays involving lysis. In order to reduce the likelihood of either degradation of WBCs or interference from unlysed RBCs (either of which would jeopardize the accuracy of the overall WBC concentration measurement), a careful combination of concentration of lysing agent, temperature control, and incubation time must be used. In some cases, the user is offered several test options with different lysing conditions, thereby allowing the user to tailor the assay to the subject patient sample. This tailoring, however, is a complex solution, which additionally either requires prior knowledge of the state of the patient, or must be used as a reflex test following a standard CBC.
Regarding the fluorescence-based approach at discriminating between RBCs and lymphocytes, the main obstacle is the measurement rate. When WBCs are measured at the same time as RBCs and platelets, the presence of RBCs sets an upper limit to the concentration that can be sent through the analyzer without incurring in coincidences at an unacceptably high rate; the dilution ratio used to achieve such concentration, in turn, limits the rate at which WBCs events are being counted; and in order to obtain the counting precision expected of the analyzer, this relatively low rate of WBC event acquisition, in turn, forces long acquisition times. For example, the concept of measuring all of the components of blood from a single sample in one pass was disclosed in U.S. Pat. No. 6,524,858. As noted in that disclosure, the method would be capable of a cycle time of 88 seconds, or about 41 CBC/hr. This throughput is far lower than that achievable by most automated hematology analyzers commercially available today, severely limiting its commercial usefulness. The CELL-DYN® Sapphire®, as another example, presently offers a test selection (requiring yet another aliquot of sample in addition to those used in the RBC/platelet assay and in the WBC assay) employing a nucleic-acid dye capable of differentiating between RBCs and lymphocytes. This test selection uses the dye primarily to differentiate between mature RBCs and reticulocytes, a subset of immature RBCs that retain dye-absorbing RNA in the cytoplasm. While it would technically be possible to count the WBCs using this same assay, as they are sufficiently differentiated by fluorescence from either RBCs or reticulocytes to obtain the desired accuracy, the relatively low concentration of WBCs in the dilution used makes it an impractical option to achieve the required statistical precision. Such a scheme would require an acquisition time of approximately 75 seconds, limiting throughput to only 48 CBC/hr. Accordingly, although this approach is theoretically feasible, a much higher throughput would be required in order for this approach to become practical commercially.
A single-dilution approach presents many attractive benefits. One of them is the elimination of multiple aliquots: This feature drastically simplifies the fluidic architecture of the system, since it requires a single container (instead of two or more) in which to mix the blood sample and the reagent solution, and a single system (such as, for example, a precision metering syringe and associated driver motor and control electronics) for measuring and delivering the reagent solution to the mixing container. It also affords an attendant reduction in the number of valves, the number of valve actuators, the number of individual segments of tubing, and the number and quantity of reagents necessary to implement the desired assay. Another benefit is the elimination of the process of lysing RBCs: This feature reduces drastically the uncertainties associated with lysis-resistant RBCs and with lysis-prone lymphocytes; it eliminates the need for the time-consuming and sensitive lysis incubation period; and, additionally, it eliminates a significant portion of the software dedicated to operate the analyzer, as previously separate test selections are combined in a single procedure. Another benefit accrues from the overall reduction in complexity of the analyzer due to the individual changes just described.
There are additional potential attendant reductions in complexity. Hematology analyzers designed for high throughput also generally include additional transducers in addition to the flow cytometer subassembly incorporated therein, such as, for example, one or more impedance transducers to count, size, and identify some subpopulations of blood cells, and a colorimetric transducer to determine the hemoglobin-related parameters of blood. A single-dilution analyzer could eliminate the need for additional impedance transducers, for a colorimetric transducer for measurement of hemoglobin, or for both impedance transducers and colorimetric transducers for measurement hemoglobin, if the analyzer were capable of achieving sufficient speed in measurement to render these deletions practical. Because the colorimetric transducer for measurement of hemoglobin requires the use of a strong lysing agent to dissolve the membranes of the RBCs (the lysing agent typically being in addition to the milder lysing agent used in the WBC assays), elimination of the calorimetric transducer for measurement of hemoglobin would also eliminate the need for an additional on-board lysing agent in addition in addition to that used in the flow cytometer subassembly. The reduction in complexity, whether from simply replacing the flow cytometer subassembly of the prior art with a single-dilution subassembly while maintaining a separate colorimetric transducer for measurement of hemoglobin or an impedance transducer or both, or from additionally incorporating all the functions of impedance transducers and colorimetric transducers for measurement of hemoglobin into the single-dilution analyzer, would result in a substantial improvement in the reliability of the instrument, because the number of parts subject to failure would be reduced, and because the number of components generating potentially damaging heat would be reduced. This improvement in reliability would likewise provide a major improvement in the instrument's service profile, with less maintenance required, fewer service calls required, and a lower cost for those calls that do occur, on account of the increased serviceability of a simplified instrument architecture, i.e., an instrument having fewer components.
All of these benefits, however, are overshadowed in the prior art by the low throughput of the disclosed method. In other words, the single-dilution feature disclosed in prior art is only one of the enabling elements of a superior analyzer. It would be desirable to enhance the single-dilution approach with a high measurement rate in order to also provide the throughput performance commonly expected of commercial hematology analyzers, and typically expected of analyzers designed for high-volume environments.